This invention relates to voltage controlled oscillator (VCO) systems and more particularly to VCO systems used in frequency memory systems.
There are presently in use several types of electronic systems whose function is to provide a continuous wave output at the same frequency as the received signal where the received signal is a short duration RF burst such as from a radar. These systems sometimes referred to as frequency memory systems must be capable of tuning to a given frequency within a fairly broad range of frequencies rapidly and accurately. One commonly used system employs a microwave delay line and a broadband amplifier, such as a traveling wave tube, in which the incoming RF burst signal is recirculated many times to produce a practically continuous output. Another approach is the use of a set-on voltage controlled oscillator (VCO) in a system where a discriminator produces a voltage related to the frequency of the incoming signal, a sample and hold circuit is used to store this voltage (memory), and a VCO is tuned by this stored voltage. The accuracy of this system is determined by the discriminator and the VCO transfer characteristics and the memory time is limited by the sample-and-hold circuit storage time and the VCO drift. In the usual VCO system, the discriminator output is amplified and shaped so as to match the discriminator to that of the VCO tuning curve and to tune the VCO to the same frequency as the incoming signal. The accuracy of the set-on voltage to tune the VCO is limited by the calibration of the discriminator and the VCO and the changes in the operating characteristics of the discriminator and the VCO caused by variation in ambient temperatures and power supplies.
In conventional set-on VCO memory systems, the incoming signal is discriminated and the resulting output voltage is stored in the memory such as the sample and hold circuit. Because the discriminator calibration is limited in accuracy and resolution, the stored voltage is inaccurate to some degree. The problem is compounded by the fact that the incoming signal will be varying in amplitude unless major limiting and leveling accessories are added to the system. Furthermore, additional error is produced by drift or instability of the VCO after the stored output voltage is applied.